1. Field of the Invention
The invention pertains to dynamic multispectral x-ray imaging of objects, such as an object, animal, tissue sample, or human body.
2. Description of the Related Art
X-rays are generated by a high voltage X-ray tube that is driven by a control unit and interconnected through cabling to a high-voltage generator. A beam of the X-rays pass through a subject body during X-ray examination and onto an X-ray detector array. The X-ray detector array may be a flat panel detector, for example as described in U.S. Pat. No. 6,403,965 issued to Ikeda et al. Other detection devices are known, for example, a charged coupled device (CCD) formed as a regular array of light sensitive cells and coupled to an x-ray attenuation layer such as a scintillator or a photoconductor, as shown and described in U.S. Pat. No. 5,454,022 issued to Lee et al. Various manufacturers produce large format CCD devices for use in X-ray detection applications. Alternatively, the X-ray detector maybe a flat-panel array with amorphous Silicon (a-Si) substrate with either a scintillator layer such as CsI, or a direct convertor material such as Cadmium Zinc Telluride (CZT), Mercuric Iodide (HgI2), Lead Iodide (PbI2), or amorphous Selenium (a-Se).
In a CCD, electric charge builds up in each element of the detector array in proportion to the integral (as function of energy) of the intensity of the X-rays impinging on the element. To take a measurement, the charge in each of the cells is set to zero (or a corresponding nominal level), the array is exposed to the X-rays, and the charge accumulates over a period of integration. At the end of the integration period, the accumulated charge is transferred to a measurement circuit in a regular and known sequence. The result of this process is a series of measurements (one for each element in the array) that relates to the magnitude of X-rays passing through, and through inference absorbed by, the subject body. Each of the elements of the detector array corresponds to a unique straight-line path from the X-ray source center through the subject body to the detector element center. Equation (P.1) shows Beer's Law, which defines the line integral of the absorption of any given path through the subject body:
                    I        =                              I            0                    ⁢                                    ∫                              spectrum                ⁢                                                                  ⁢                E                                      ⁢                          exp              ⁢                              {                                                      ∫                    0                    d                                    ⁢                                                            μ                      ⁡                                              (                                                  x                          ,                          E                                                )                                                              ⁢                                          ⅆ                      x                                                                      }                            ⁢                              ⅆ                E                                                                        (                  P          ⁢          .1                )                            where I=Output X-ray intensity level;        I0=Incident X-ray intensity level;        E is one of the energies comprising the x-ray spectrum;        μ(x)=Attenuation at a point on the X-ray path through the subject; and        [0,d] represent the length of the X-ray path under consideration.        
Modern digital x-ray systems often include an automated software analysis package. Breast cancer screening investigations today provide the best case study of computer assisted diagnosis (CAD) approaches. The potential of CAD to improve chest radiography, chest CT, and other medical examination accuracy is the subject of much on-going research. Current CAD products and algorithms tend to have low specificity when used alone without the judgment of a radiologist, as applied to breast cancer screening, and help mostly less experienced readers. Yet several studies have suggested that CAD can improve upon a radiologist's ability to detect and classify lesions in mammography and such findings are likely in chest imaging and in other applications as well. Automatic detection of lung nodules is the most studied problem in computer analysis of chest radiographs. Nodules typically present as relatively low-contrast densities within the lung fields. The challenge for CAD in chest radiography is to distinguish true nodules from overlapping shadows of ribs and vessels. Computer approaches might prove essential to the practicality of measuring two of the best predictors of module malignancy: nodule size and growth rates.
Patient and population X-ray exposure and dose concerns have been heightened in recent years. In particular, computed tomography (CT) and MDCT examinations have increased in number and frequency; the annual number of CT examinations in the United States increased almost 10-fold in less than 20 years. In the decade preceding the introduction of MDCT (1980 to 1990), the number of CT exams increased from 3.6 million to 13.3 million. In the United States, CT accounts for only 11 percent of the total X-ray examinations, yet it contributes about 66 percent of the total delivered dose. Worldwide the corresponding percentages are approximately 5 and 34 percent. In lung imaging, CT has demonstrated significant sensitivity, but its relatively low specificity has raised concerns about the likelihood of significant changes in patient outcomes.
Dose concerns, the worldwide health impact of tobacco use, and the frequency of chest X-ray examinations all contribute to the urgency and significance of finding low-dose means of detecting lung cancer and other diseases. By intrinsic design, and for a given X-ray absorption technology, a narrow-beam scanning system provides the optimal means to lower patient dose, due to the absence of a Bucky grid. For a given detector technology and X-ray beam spectrum, a scanning approach offers the best DQE possible, provided the beam penumbra is utilized and the scan mechanism is appropriately designed to minimize system losses.
As illustrated, FIG. 1 shows one prior art digital X-ray system. Projection imaging or CT system 10 includes an X-ray source 12, an X-ray detector array 14, and a subject body 16. X-ray source 12 and X-ray detector array 14 are maintained in a fixed geometric relationship to one another by means of a gantry for the duration of an exposure (not shown) permitting selective rotation 18 about a fixed axis 20 that is generally coaligned with body 16 or another axis of rotation. Each X-ray exposure is taken from a nominal position causing X-rays 21 to pass through section 24 of subject body 16. The detector array 14 provides electronic signals commensurate with the absorption of X-rays 21 after passage through section 24, and these signals constitute measurements of properties encountered in section 24. The measurement data are representative of the integral of the body X-ray linear attenuation coefficients along paths 21.
Computing equipment 22 receives signals from the detector array 14 and processes these signals. By application of such mathematical processes as projection data calibrations, corrections, and image reconstruction, the signals are converted to an image of section 24. The image can then be examined, for example, on a video display 26.
Subject body 16 may be placed on a moveable platform 28, which is moved by an electric motor 30 under the control of the computing equipment 22. As desired, additional images may be obtained from the subject body 16, e.g., at a different location, at different platform locations relative to X-ray source 12 and detector array 14.
In another mode of operation, the projection imaging system 10 of FIG. 1 may be used to provide CT scans. X-ray source 12 and X-ray detector array 14 are maintained in a fixed geometric relationship to one another by means of an automated gantry (not shown) as they are both rotated 18 about a fixed axis 20 generally coaligned with body 16. In this rotation, X-ray detector array 14 and X-ray source 12 are continuously rotated about the body 16 to complete a circle about body 16. The X-ray exposure typically is continuous and a series of projection measurements are acquired at a multiplicity of positions or projection angles around the body. After completion of a source rotation, the subject position may be incremented by a pre-determined amount, and the examination continues.
These measurements are collected by computing equipment 22. By application of mathematical processes, this set of measurement results is converted to tomographic map of a section 24 of body 16. The tomograph can then be examined, for example, on a video display 26.
Subject body 16 is generally placed on a moveable platform 28, moved by an electric motor 30 under the control of the computing equipment 22. As desired, a further scan section 24 may be made, e.g., at a different location, at different platform locations relative to X-ray source 12 and detector array 14, to generate a more complete 3D representation of subject body 16. Scanning is often performed in a “helical” mode, where platform 28 advances continuously as X-ray detector array 14 and source 12 rotate. The timing of the detector cell samplings determines the geometry of the X-ray paths through the body. The measurement data are representative of the integral of the body X-ray linear attenuation coefficients along these paths. In such modes the X-ray source may be energized continuously during the examination time.
It is also appreciated that in a CT system, the subject table may be advanced while the source and detector remain at a fixed spatial position (a “Scout” mode). In such a situation a fixed point in the subject body is projected through the various detector rows as a function of time.
Unfortunately, one problem with system 10 is that the tomographic map of section 24 can be used to identify relatively few constituents of body 16, and measurements lack quantitative accuracy.
The X-ray source 12 and X-ray detector array 14, as connected by the gantry, define an imaging chain, and are maintained in a defined geometric relationship to one another by means of a gantry. The source-to-detector distance may be varied depending on the examination, and the angle of the imaging chain can be adjusted with respect to the body to be imaged.
System 10 may be adapted for use in security applications, such as airport security screening of luggage, or industrial applications where it is desirable to X-ray a part or package.